In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of an optical fiber comprising a transmission link of a WDM fiberoptic network. At a receiver end of the transmission link, the optical channels are separated, whereby each optical channel can be detected by an optical receiver.
While propagating through an optical fiber, light loses power. This power loss is well understood and is related to the physics of propagation of light in the fiber. Yet some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end.
An erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical amplifiers employed in fiberoptic networks. A single EDFA module can amplify about a hundred optical channels at a time, thus providing significant cost savings. One of the main components of an EDFA is a length of an active optical fiber having a core doped with ions of a rare earth element erbium. The erbium doped fiber (EDF) is optically pumped by using a suitable pump, such as a diode laser, so as to create a population inversion between energy states of the erbium ions comprising a gain medium of the EDF. Once the population inversion is created, the gain medium begins to amplify an optical signal propagating along the core of the EDF. The gain medium is characterized by a wavelength-dependent gain coefficient. During the amplification process, the optical power of the pump is absorbed by the gain medium, which simultaneously amplifies all the optical channels present in the optical signal. The amplification coefficient of a particular channel depends on the input optical power and on the optical power of the pump. When the number of optical channels changes suddenly, for example, due to adding, dropping, or routing of some of the optical channels, the input optical power changes stepwise, which results in a change of the gain coefficient of the gain medium of the EDF. The gain coefficient change impacts output optical power of the rest of the optical channels.
Most optical amplifiers of the prior art have a gain stabilization circuit that reacts to changes of input optical power by changing the pump optical power. For example, in U.S. Pat. No. 6,989,923 by Stentz, an apparatus for automatically controlling gain of an optical amplifier is disclosed. The apparatus of Stentz generates a first control signal from a feed-forward control circuit and a second control signal from a feedback control circuit. The optical power of the pump is adjusted in accordance with both control signals. Similarly, in U.S. Pat. Nos. 6,246,514 and 6,522,460 by Bonnedal et al., the feed-forward and feedback controls are combined, and in addition, pilot tones are used to measure the amplifier gain. In one embodiment, an EDFA controller of Bonnedal et al. calculates the number of channels present in the input optical signal and adjusts the optical power of the pump accordingly.
Disadvantageously, the accuracy of transient control, that is, the degree to which transient fluctuations of optical channel power may be suppressed, is limited by the temporal dependence of the EDF optical gain. Even when the optical power of the pump increases instantaneously, the EDF optical gain does not. There is a certain delay of the EDF gain growth following the pump increase, which is related to the rate of populating an excited meta-stable energy level 4I11/2 of erbium ions. Similarly, when the power of the pump decreases, or when the input optical power increases, the gain does not decrease instantaneously. The gain in fact decreases at a rate of change of the population inversion in the EDF gain medium. As a result, a transient change of the gain coefficient and, consequently, a transient change of output optical power is produced.
These transient fluctuations of optical power of a signal can grow in magnitude as the signal propagates along a transmission link containing many EDFAs, which can ultimately lead to a loss of information and even to a loss of network stability and/or to damage of optical receivers. To avoid stability loss or damage to network components, it is imperative that transient changes of optical power of propagating signals be kept below a certain acceptable level.
A method for adaptively controlling an optical gain in an EDFA has been described in U.S. Pat. No. 6,894,832 by Aweya et al. In the method of Aweya et al., the temporal behavior of the optical gain in the EDF is approximated by using a so-called reference model. The reference model of Aweya et al. is a dynamic model used for computing a reference value of the output optical power corresponding to the input optical power and target optical gain of the EDFA. The reference value is compared to a measured value of the EDFA output optical power, and the optical power of the pump is adjusted so as to bring the measured value of the EDFA output optical power to the computed reference value. An adaptation mechanism is described for adjusting a ratio between the signal from the reference model, the feedback signal, and the feed-forward signal, wherein all three signals are used to adjust the optical power of the EDFA pump.
Disadvantageously, the method of Aweya et al. is computation intensive, which can lengthen the response time of a corresponding control apparatus. Transient variations of the output optical channel power can occur in a sub-microsecond time domain. Given the amount of the computations required to implement the method of Aweya et al. for controlling the optical gain of an EDFA, the sub-microsecond response time may be difficult to achieve in combination with the required degree of transient suppression.
An apparatus and a method for controlling gain in an optical amplifier by accounting for transient changes of energy levels population in the EDF gain medium has been described by Lelic in U.S. Pat. No. 6,900,934. The method of Lelic involves real-time tracking of the population of an excited energy state of erbium by measuring a residual pump power, that is, by measuring the optical power of the pump light which has not been absorbed in the EDF. Disadvantageously, the apparatus of Lelic comprises a complicated digital processor, as well as an optical tap, an optical filter, and a photodetector dedicated to measuring the residual pump optical power.
Another known approach to reducing transient variations of gain of an optical amplifier consists in stabilizing overall power of the input optical signal before it reaches the optical amplifier. For example, Cordina in U.S. Pat. No. 6,668,137 discloses a power control apparatus for stabilizing overall optical power of a signal propagating in an optical fiber. Referring to FIG. 1, a power control apparatus 100 of Cordina includes an optical tap 102, a photodiode 104, an optical delay element 106, a variable optical attenuator (VOA) 108, and a controller 110. In operation, the controller 110 receives a signal from the photodetector 104 and adjusts the VOA 108 to keep the output optical power constant. The delay element 106 is required to compensate for a finite response time of the VOA 108 and the controller 110. The Cordina apparatus 100 enables power transients to be detected in time for a “pre-emptive” control, to suppress transients which are too fast to be suppressed by the above described conventional control of optical amplifiers.
Lundquist et al. in U.S. Pat. No. 7,483,205 discloses a similar apparatus. The apparatus of Lundquist et al. includes a variable optical attenuator, an optical power sensor disposed upstream of the variable optical attenuator, and a control loop configured to stabilize optical power after the variable optical attenuator. The control electronics and the variable optical attenuator of the Lundquist apparatus must be very fast to be able to react to sub-microsecond input optical power changes, which increases the cost of the Lundquist apparatus.
Furthermore, a common drawback of optical power stabilizers of Cordina and Lundquist et al. is that optical power stabilizers of Cordina and Lundquist et al. achieve power stabilization by attenuating the optical signal before the amplifier, thus lowering achievable optical signal-to-noise ratio. The signal-to-noise ratio is lowered because the amplified spontaneous emission (ASE) noise of the optical amplifier is not attenuated by the variable optical attenuator, while the incoming optical signal is.
In view of the foregoing, there is a need to provide an apparatus and a method for reducing optical transients, that would overcome the above described prior-art shortcomings of slow response time, complexity of the control circuitry, and varying per-channel optical power. The present invention allows one to considerably reduce transient variations of output power of optical amplifiers by using a simple control circuit, which can be easily added to existing optical amplifiers of different designs.